MOSFETs (Metal Oxide Semiconductor Field-Effect Transistors), in particular power MOSFETs, are widely used as electronic switches for switching electrical loads or as electronic switches in all types of switching converters. A power MOSFET typically includes a drain region, a drift region adjoining the drain region, and a source region, each having a first conductivity type, and a body region arranged between the drift region and source region of a second conductivity type. A gate electrode serves to control a conducting channel in the body region between the source region and the drift region. The source region is electrically connected to a source electrode which is also connected to the body region, and the drain region is electrically connected to the drain electrode. The MOSFET can be switched on and off by applying a suitable drive potential to the gate terminal.
In a specific type of MOSFET, a field electrode is arranged in the drift region and is dielectrically insulated from the drift region by a dielectric layer. This field electrode is usually coupled to the source electrode. By virtue of the field electrode, the drift region can be more highly doped than in conventional MOSFETs, resulting in a reduced on-resistance, at a given voltage blocking capability. A reduced on-resistance results in reduced ohmic losses of the transistor device.
The field electrode, however, increases the output capacitance of the transistor device, which increases capacitive losses that may occur in operation of the transistor device.
There is, therefore, a need to provide a transistor device with a field electrode that can be optimized in terms of capacitive losses and ohmic losses.